For mass spectrometric methods in biochemistry, in particular in genetic and protein research, the amount of substance used by these methods is a decisive factor. In order to obtain a mass spectrum from a few attomols of a substance (1 attomol=600,000 molecules), it is necessary to maximize the ion yield of the ionization process and to minimize the ion losses at all stages from ion generation to ion measurement. The yield of every stage must be optimized.
When RF quadrupole ion traps are used as mass spectrometers, the process of capturing externally generated ions in the ion trap usually results in a widely unsatisfactory yield. Hitherto only three to five percent of the ions being continuously produced are trapped, the remainder is usually lost.
The intermediate storage of the ions in an RF ion guide already represents a great improvement as far as the optimization process is concerned. It is thus possible to temporarily store ions from a continuously operating ion source in such a way that the quadrupole ion trap is only loaded with ions during a relatively short filling time. During the protracted analysis time, on the other hand, the ions are temporarily stored and thus collected. In particular, the ions in the RF ion guide can be decelerated to thermal energies (“thermalized”), thus improving the capturing process in the quadrupole ion trap. The RF ion guide consists usually of a system of parallel rods, arranged on a virtual cylinder, to which the two phases of an RF voltage are alternately applied. Quadrupole, hexapole and octopole systems have proven successful for this. It is also possible to use other types of RF ion guides such as double helices or ring systems to which an RF is applied.
However, even with this intermediate storage of ions, the yield of the capturing process of the ions which are injected into the quadrupole ion trap is still unsatisfactory.
At present, there is still relatively little known about the mechanism by which ions are captured in the quadrupole ion trap. Research, encompassing experiments on ion traps as well as computer simulations, has shown that ions can only be trapped in an extremely short phase interval of a few percent of the complete RF period. The length of the capturing interval is strongly dependent on the injection energy of the ions and weakly dependent on the pressure of the collision gas in the ion trap. In the remaining phases of the RF period (outside the phase interval in which the ions can be captured), the ions may be reflected at the entrance to the quadrupole ion trap (“reflection interval”), because they encounter an opposing strong high-voltage field inside the ion trap. Otherwise, they experience an accelerating suction field (“transverse interval”), are accelerated in the ion trap towards the end cap opposite the entrance, traverse the ion trap without being sufficiently decelerated and strike the end cap. They are lost for further use by being discharged at the end cap. Depending on the strength of the momentary suction field at the entrance, i.e., on the phase of the RF voltage, the traversing process may take place in less than one RF period, but it also may take around ten to twenty RF cycles. If the traversing process is slower than that, the collision gas decelerates the ions, and capturing will be achieved. The operating pressure of damping gas which is favorable for the operation of quadrupole ion traps (usually helium or nitrogen) has free path lengths of the order of magnitude of one ion trap diameter in the injection direction, and is therefore not sufficient to decelerate ions during their first traverse.
U.S. Pat. No. 5,739,530 describes how the ion yield can be improved by forming packets of ions for the injection by means of a switchable ion lens. The ions are then injected in individual packages at the phase interval favorable for capture. With the extremely short capturing interval which usually prevails, this method, however, fails because the mass-dependent flight velocities in the injection lens mean that only ions within a narrow mass range can be injected for capture in the short interval. It has not yet proved possible to really use the basic idea of this patent and, despite intense efforts, the yield of the capturing process for ions injected into the quadrupole ion trap has not exceeded five to ten percent of the available ions up to now.